Whats wrong with my with strlen and substr PHP

I created a session called product_ .
When i click in my add to cart button, my

$_SESSION['product_' . $_GET['add']]

will be like this product_171
the 171 is the id from a product that i have.
I printed out my query to see what's going on and my item_id is not echoing 171
it's sendind 17 not 171.
i tryed to change the numbers but it didn't work.

$query = "SELECT * FROM gallery2 WHERE id =".escape_string($item_id)."";
print_r($name); echo' <strong>this is the name</strong></br>';
print_r($item_id); echo' <strong>this is the id without the name -product_</strong></br>';...

this is what i'm getting

product_171 this is the name

17 this is the id without the name -product_
why im not getting my full id?